class_docstr: Create an object holding a new letterboxed image as `image` attribute.
  
  Letterboxing is defined as scaling the input image to fit inside the
  desired output image frame (letterbox) while preserving the aspect
  ratio of the original image. The pixels that are not filled with the
  original image pixels become 127.
  
  The created object also provides a functionality to convert bounding box
  coordinates back to the original image frame.
  
  Args:
  
      image (numpy.ndarray): An uint8 3-channel image 
      height (int): Letterbox height
      width (int): Letterbox width
code: def __init__(self, image, height, width):
      self.height, self.width = height, width
      self.im_h, self.im_w = image.shape[:2]
      self.image, self.new_w, self.new_h = letterbox(
          image, height, width
      )
  
file_path: nnabla/python/src/nnabla/models/object_detection/utils.py
imports: import numpy
  from nnabla.models.object_detection.draw_utils import DrawBoundingBoxes
  from nnabla.utils.image_utils import imresize
label_desc: Use image node_id 1 to create a new LetterBoxTransform object from inherited base classes: object with example: obj = LetterBoxTransform(image, height, width)